For environment protection purposes, in gas turbine engines, strict environmental standards are set in a composition of combustion exhaust gas emitted. It is required to reduce harmful substances such as nitrogen oxide (hereinafter expressed as NOx) from the exhaust gas. In large-sized gas turbines and engines for aircraft, a pressure ratio tends to be set high, because of requirement of low fuel consumption and a high power output. Correspondingly, air at an entrance of a fuel supply device tends to be placed in higher temperature and higher pressure conditions. With an increase in the temperature of the air at the entrance of the fuel supply device, a combustion temperature rises, which may undesirably result in an increase in the amount of NOx in the exhaust gas.
Under the circumstances, in recent years, there has been proposed a combined combustion method in which two combustion methods, i.e., a lean pre-mixed combustion method which can reduce the amount of NOx generation effectively, and a diffusion combustion method which has high ignition performance and high flame stabilizing performance are combined. In the lean pre-mixed combustion method, air and fuel are pre-mixed and an air-fuel mixture with a uniform fuel concentration is combusted. In this method, a combustion zone in which a flame temperature is locally high does not exist. In addition, the flame temperature can be lowered as a whole because of lean fuel. Therefore, the lean pre-mixed combustion method has an advantage that the amount of NOx generation can be reduced effectively. However, in the lean pre-mixed combustion method, since a great amount of air and fuel are mixed uniformly, a fuel concentration in a combustion zone is very low, which degrades combustion stability particularly under a low load state. By comparison, in the diffusion combustion method, the fuel and the air are combusted while diffusing and mixing them. This method has advantages that a flame is less likely to vanish even during the low load state, and the flame stabilizing performance is high. Therefore, in the combined combustion method which is a combination of the lean pre-mixed combustion method and the diffusion combustion method, combustion stability in a diffusion combustion zone can be maintained during starting or during the low load state, and the amount of NOx generation in a lean pre-mixed combustion zone can be reduced during a high load state.
A fuel supply device according to the combined combustion method includes pilot burners which spray a fuel so as to form a diffusion combustion zone within a combustion chamber by the diffusion combustion method, and main burners which supply an air-fuel mixture containing pre-mixed fuel and air so as to form a pre-mixed combustion zone within the combustion chamber by the lean pre-mixed combustion method. In this fuel supply device, fuel is supplied only to the pilot burners during starting or during the low load state, while the fuel is also supplied to the main burners in addition to the pilot burners during the high load state. When the gas turbine engine transitions from the low load state to the high load state, the fuel supply device controls a fuel flow rate so that a fuel dividing ratio between a fuel supplied to the pilot burners and a fuel supplied to the main burners changes gradually from 1:0 to, for example, 1:9, while maintaining a proper value of the ratio for achievement of stable combustion and reduction of NOx.
In a conventional fuel supply device by the combined combustion method, to perform the above complicated control, flow control valves are respectively provided in a pilot fuel passage through which the fuel is supplied to the pilot burners and a main fuel passage through which the fuel is supplied to the main burners, and a controller controls these flow control valves (Patent Literature 1).
However, if the flow control valves are provided in these two fuel passages, respectively, as described above, a ratio of the weight and cost of the flow control valves and the controller with respect to weight and cost of an overall engine increases especially in a small-sized gas turbine for aircraft, and its influence is non-negligible, although the ratio is differed between a gas turbine for aircraft and a gas turbine for industry, or between a large-sized gas turbine and a small-sized gas turbine. This precludes application of the combined combustion method which requires an additional fuel control system (flow control valves and controller) to the small-sized gas turbine for aircraft. Further, the addition of the fuel control system causes an increased weight and a complicated structure.
Under the circumstances, the applicant proposed a system in which a fuel divider is provided between a fuel passage through which the fuel is supplied to the pilot burners and a fuel passage through which the fuel is supplied to the main burners, and a collecting fuel passage through which the fuel is supplied to these fuel passages, and a controller controls the fuel divider (Patent Literature 2).